This invention relates to passive liquid crystal displays of the type which employ a backlight for illuminating the display in the dark.
Passive liquid crystal displays (LCDs) provide advantages over those displays having light emitting diodes (LEDs) in that information is always being displayed and is readable even in bright sunlight. However, LCDs depend on ambient light and are not visible in a dark environment unless some form of support lighting system is implemented to backlight the display. The source of energy for a support lighting system is typically provided by one or more batteries.
Incandescent light bulbs are used as a light source for backlighting LCDs and can be made sufficiently small so as to minimize the amount of current drawn from the energy source. An incandescent backlight for illuminating a liquid crystal display in a dark environment is described in U.S. Pat. No. 3,712,047 issued to Gerard. However, major problems concerning the use of incandescent light bulbs for backlighting LCDs include inadequate dispersion and diffusion of light, high power consumption, and poor quality filaments used in such bulbs. Poor quality filaments generally have short lifetimes. Other problems associated with incandescent backlights is the difficulty of properly encasing each filament in a glass enclosure especially of the size required in small LCDs such as those used in timepieces.
Radioluminescent materials have been used as a source of light for backlighting LCDs. Radioactive materials such as tritium or promethium generate radiant energy in the form of electrons which are permitted to bombard radioluminescent materials. The electrons are produced by the deterioration of heavy hydrogen atoms. Radioluminescent material includes a variety of phosphors which reradiate some of the absorbed energy as visible light. The emitted light has a wavelength that is characteristic of the particular phosphor material used. The phosphor therefore is actually a source of light that provides some advantages over incandescent light bulbs. The fact that no external energy source is needed to provide for continuous light output and the fact that no filaments burn out or no glass enclosures break are just some advantages provided by the use of radioluminescent materials over incandescent light bulbs. However, radioactive material such as tritium can produce secondary radiation having very short wavelengths thereby creating a potential health problem. Tritium backlights are, therefore, generally placed in metal containers which act as shields against excessive emissions of radiation. Nevertheless, tritium backlights, which are sealed glass bulbs internally coated with a phosphor and filled with tritium gas, can still emit high levels of radiation depending upon the size of the backlight. For example, a two-bulb tritium light emits three times the radiation of a three-bulb tritium light since, in order to obtain high intrinsic brightness, each bulb in the two-bulb light is pressurized. Furthermore, a smaller tritium backlight helps to reduce cost since tritium is expensive and can account for the greatest part of the cost of most tritium lights. Since an intrinsically brighter two-bulb light is frequently used as a backlight in LCDs, and since a two-bulb light emits a considerable amount of radiation, a two-bulb tritium backlight poses a potential health hazard. Another problem associated with tritium backlighting systems is the generally shortened bulb life due to the entrapment of residual moisture during bulb manufacture. The residual moisture reacts with the tritium to form non-radioactive hydrogen peroxide. Also, as suggested above, the cost of a tritium backlighting system prohibits its acceptance as a replacement for incandescent backlighting systems. Light produced by radioactive light sources are discussed in U.S. Pat. No. 3,701,900 issued to Thuler; U.S. Pat. No. 3,566,125 issued to Linhart et al.; U.S. Pat. No. 3,204,103 issued to Johnson et al.; U.S. Pat. No. 4,126,384 issued to Goodman et al.; and U.S. Pat. No. 4,221,112 issued to Enomoto et al.
Attempts have been made to use fluorescent materials as a source of light for backlighting LCDs. The fluorescent material is activated by ambient light during the daytime to emit visible light during hours of darkness. However, light emitted from fluorescent materials is not bright enough for any length of time to be useful for most backlighting applications. Furthermore, fluorescent materials need to be frequently rejuvenated by ultra-violet and infrared light. However, ultra-violet and infrared light transmission through the LCD is impaired and cannot efficiently reach the underlying fluorescent material thereby reducing the level of brightness attainable by a backlight of this type. U.S. Pat. No. 4,276,633 issued to Takami et al. discloses a self-luminescent light source comprising a fluorescent substance.
Transflectors have been implemented, especially those with high transmissive properties, for efficiently transmitting light from a light source through the LCD for improving the viewability of the display at night. U.S. Pat. No. 4,196,973 issued to Hochstrate, for example, discloses a transflector for illuminated electrooptic displays. A standard backlighted LCD typically includes a liquid crystal cell having image-forming electrodes, upper and lower polarizers on opposite sides of the cell, and a transflector behind the lower polarizer. A light source is positioned behind or at the side of the transflector for directing light therein. The transflector functions essentially as a one-way mirror so that the display can operate in the reflective mode in ambient light and in the transmissive mode at night with the aid of backlighting by the light source. In daylight, ambient light enters the display and passes through the liquid crystal cell, where it may be optically altered and then is reflected from the transflector back toward the observer. Light provided by the backlight is generally reflected due, in part, to a plurality of facets on the back of the transflector diffuser layer and, in part, to the total transflector reflector layer. At night, the transflector receives light from the light source and transmits it upwardly toward the observer by light scattering, diffusion, and reflection within the transflector. As a result of the light diffusion effected by the transflector, the display characters are more or less uniformly illuminated. A typical transflector may comprise silvered glass or glass with a gray filter. However, other materials which are translucent to light have also been widely used. For example, styrofoam of white, gray, or silver hue, polystyrene and polypropylene in thin layers or sheets.
Choice of the correct transflector material is the most critical element in prior art backlighted LCDs since it must provide optimal light output in both the light transmissive and light reflective modes, i.e. during both nighttime and daytime hours, Ideally, the transflector should have highly reflective properties under all levels of ambient light conditions during daytime hours and highly transmissive properties during nighttime hours. However, improving the nighttime viewability of a display by improving the transmissive properties of its transflector can adversely affect the quality of the daytime viewability of the display by reducing, for example, the contrast between the images formed and the background.
When a tritium backlighting system is implemented with a display having a transflector, much of the light provided by the tritium is lost in the transflector thereby reducing the contrast between the images formed and the background. Therefore, any attempt to improve the transmission of light emitted from a tritium backlighting system for nighttime viewing of the LCD reduces the reflectivity (reflectance) of the transflector and diminishes the quality of images formed when viewed in the daytime. Selection of a transflector that does not adversely affect reflectance and the contrast ratio of the LCD is particularly critical when using a tritium backlighting system.
Also, when an incandescentt backlighting system is implemented, the selection of a transflector having an optimal diffuser layer is critical in obtaining efficient light transmission and reflection for high-quality LCD viewability. The thickness of the diffuser layer affects the light dispersion properties of the display such that the thicker the diffuser, the better the light dispersion and the better the viewability. However, in small LCDs, a thick transflector diffuser layer may, in some cases, be undesirable since it would add to the thickness of the display.
Diffuser layers in transflectors present additional problems especially when used with incandescent light bulbs. For example, strain patterns generally form in the diffuser layers and become illuminated with polarized light. This gives rise to background shadows on the display and non-uniformity of light dispersion. To attempt to reduce the amount of strain patterns in the diffuser layer, careful transflector molding techniques are required in which mold time and temperature are critical parameters. Another critical feature of prior art transflectors is the faceting pattern provided on one surface of the diffuser layer. Also, optical coupling of an incandescent backlight bulb to the transflector diffuser layer is critical in that the center line of the filament must match the center line of the diffuser layer to provide optimum light dispersion. Optimal light dispersion is possible when the bulb diameter is equal to the diffuser layer thickness. However, with the implementation of larger incandescent bulbs and thinner LCDs, matching center lines and allocating space for the incandescent backlighting system becomes a problem.
Another type of backlight that can be used to provide illumination of liquid crystal displays utilizes electroluminescent material. However, electroluminescent backlighting systems have not been seriously considered for backlighting LCDs in the past because of the excessive voltages required to operate the system, i.e. generally over 50 volts. Also, an electroluminescent backlight, when used with a transflector, produces poor daylight LCD viewability since the reflective properties of the transflector are not as good as the reflective properties of, for example, metal mirrors. U.S. Pat. No. 4,238,793 issued to Hochstrate discloses an electroluminescent panel for use as a backlight in an electrooptical display.
U.S. Pat. No. 4,208,869 issued to Hanaoka shows a liquid crystal display device equipped with an illumination device. The illumination device is an EL sheet disposed at the back of the display. The EL sheet includes a transparent or light-diffusive semitransparent substrate with a thin film which serves as an electrode, and a metallic sheet. Between the light-diffusive substrate and metallic sheet is disposed electroluminescent material. The EL sheet has a reflective surface for the purpose of reflecting available light. The reflective surface offers diffused reflection due to the use of zinc sulfide crystals as the main ingredient of the electroluminescent material. Furthermore, it is suggested that a layer be interposed between the liquid crystal display and the EL sheet which permits both diffusion and transmission of light and which may be employed as a supporting medium and incorporated in the laminated structure of the device. This layer would appear to be a transflector. There is no discussion of reducing the amount of phosphor in the electroluminescent material to provide for light diffusion for improving the daytime viewability of the liquid crystal display. Also, there would appear to be no light reflection from the metallic sheet since the metallic sheet is coated with a non-transparent barium titanate film.
Other pertinent patents include U.S. Pat. No. 3,869,195 issued to Aldrich which discloses a liquid crystal display and a segmented electroluminescent source of backlighting; U.S. Pat. No. 4,291,947 issued to Cirkler et al. which discloses the process for making a passive LCD; U.S. Pat. No. 3,613,351 issued to Walton which discloses an LCD and a power supply system for a timepiece; U.S. Pat. No. 4,128,312 issued to Lim et al. which discloses an LCD containing a high reflectivity cathode; U.S. Pat. No. 3,837,729 issued to Harsch which discloses an LCD utilizing a reflector assembly on one side of the liquid crystal material; and U.S. Pat. No. 3,728,007 issued to Myrenne et al. which discloses a reflective liquid crystal display containing a mirror.
Therefore, it is an object of this invention to improve the daylight viewability of backlighted LCDs without employing a transflector.
It is an object of this invention to use an electroluminescent backlighting system for an LCD in which the LCD exhibits improved viewability especially during daylight hours.
It is also an object of this invention to reduce the number of critical parameters required to achieve optimal viewability of backlighted LCDs during daytime and nighttime hours.
It is a further object of this invention to implement the electroluminescent material both as a light source and as a light diffuser.
Another object of this invention is to obtain improved viewability and image contrast by using a minimum number of components for the backlighted LCD.